1. Field of the Invention
Aspects of the present invention relate to a handover in a mobile communication system. More particularly, aspects of the present invention relate to a method and system for improving a call drop caused by a Radio Link Failure (RLF) before handover triggering in a Long-Term Evolution (LTE) communication system.
2. Description of the Related Art
A method of deploying several small-sized multi-cells has been introduced to satisfy requirements of a high data rate and to reliably provide various services in a next generation network cellular system. Due to such advantages, research and standardization work on combining an existing macro-cell and a small-sized femto-cell are currently being conducted in, among others, the 3rd Generation Partnership Project (3GPP) and Institute of Electrical and Electronics Engineers (IEEE) 802.16 standards.
When managing a Base Station (BS), a Self-Organizing Network (SON) is used to configure a network in a more reliable and effective manner. In particular, as it becomes clear that mobile communication systems will employ a femto-cell and a next-generation communication technique, there is an increasing interest in SONs including an automation function, such as self-configuration or self-optimization. This is because cell planning and the like cannot be performed in advance, because nodes (such as the femto-cell) are not installed in an optimal position defined by a service provider. Rather, the nodes are installed by a user, and thus the nodes have to perform optimization by themselves by detecting an environment and gathering information.
Therefore, in an SON environment, an information exchange operation for having a relation with neighbor BSs, i.e., an Automatic Neighbor Relation (ANR) function, is generally necessary for an update process performed when a new BS is added and an old BS is released. The ANR function is basically performed using a User Equipment (UE).
FIG. 1 illustrates an exemplary scenario of a Radio Link Failure (RLF) before handover triggering (hereinafter, referred to as “RLF_before_HO”) in a mobile communication system of the related art.
Referring to FIG. 1, an Element Management System (EMS)/SON server 100 performs network management, and controls an update process when a new BS (also referred to as an evolved Node-B (eNB)) is added and an old BS is released (such control is referred to as ANR function control).
An eNB A 110 is connected with a User Equipment (UE) 130 via a link 112. When the UE 130 moves to an eNB B 120 quickly, a link 116 between the eNB A 110 and the moved UE 130′ is disconnected before handover triggering is generated, and the moved UE 130′ intends to establish a link 118 by transmitting a Radio Resource Control (RRC) re-establish message to the eNB B 120.
In an SON environment, an Automatic Neighbor Relation (ANR) update function between eNBs may be necessary. An ANR configuration function detects a new neighbor cell using a UE measurement report and automatically adds a Neighbor Relation (NR). An ANR optimization function provides an NR addition/deletion function and a prioritization function based on a UE measurement result (e.g., signal strength of a neighbor eNB), a HandOver (HO) Key Performance Indicator (KPI) (e.g., handover attempt rate/success rate), and Radio Resource Management (RRM) information, and thus provides an optimal Neighbor Relation Table (NRT) management function in an automated manner.
As described above, when the UE measurement report is received, each eNB may be added to the NRT. However, in case of the RLF_before_HO, the link between the UE 130 and the serving eNB 110 is disconnected before the UE measurement report is received, and thus the serving eNB 110 cannot add the target eNB 120 to its NRT (in this case, the target eNB 120 is not included in the NRT of the old serving eNB 110). Further, since the RLF_before_HO is not recognized as an handover from the perspective of the serving eNB 110 (that is, the serving eNB 110 cannot know that the UE 130 unlinked from the eNB 110 attempts a handover to the target eNB 120), this case is not considered in the HO KPI, and is eventually not considered in determination of an NRT priority. As a result, the target eNB 120 may be deleted from the NRT of the serving eNB 110 (in this case, the target eNB 120 is included in the NRT of the old serving eNB 110).
FIG. 2 is a graph illustrating handover triggering according to the related art.
Referring to FIG. 2, received signal strength received by a UE from a serving eNB is indicated by 202, and received signal strength received by the UE from a target eNB is indicated by 204. Handover triggering is generated at an instant 210 at which the received signal strength received by the UE from the target eNB becomes greater than the received signal strength received by the UE from the serving eNB.
In practice, a hysteresis value (e.g., an offset value) is used to mitigate a handover ping-pong effect. As a result, the received signal strength received by the UE from the serving eNB is offset to a curve 212, and the received signal strength received by the UE from the target eNB is offset to a curve 214. Therefore, actual handover triggering is generated at an instant 220 at which the received signal strength received by the UE from the target BS becomes greater than the received signal strength received by the UE from the serving eNB.
Consequently, even if the UE exists in a handover area, the signal strength between the UE and the serving eNB becomes less than a signal strength threshold 200 at which an RLF occurs, and thus a link established between the UE and the serving eNB is disconnected at the instant 220 before handover triggering is generated.
In this case, a handover event triggering parameter (i.e., a handover triggering offset value based on the handover ping-pong effect, referred to as a Cell Individual Offset (CIO)) is incorrectly set, and thus a previous connection established to an eNB (e.g., the serving eNB) before the UE measurement report is received is disconnected. If the RLF occurs by late handover triggering caused by fast movement of the UE, the serving eNB should modify a handover parameter (i.e., the CIO) for the target eNB to handle this problem.
Accordingly, there is a need for a method and system for improving a call drop caused by a link failure before handover trigging in a mobile communication system.